Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. The commercially most important form of sputtering is plasma sputtering using a magnetron in back of the sputtering target to increase the density of the plasma and increase the sputtering rate. A typical magnetron includes a magnetic pole of one magnetic polarity surrounding another magnetic pole of the opposed magnetic polarity. A gap of nearly constant width and forming a closed loop separates the two poles and sets up a closed plasma track adjacent the sputtering face of the target.
Magnetron sputtering was originally used to deposit a nearly planar and relatively thick layer of a metal such as aluminum, which was thereafter etched into a pattern of horizontal interconnects. A typical magnetron used for this type of sputtering has a relatively large kidney shape with the closely adjacent poles positioned near the periphery of the pattern. The magnetron extends from about the center of the target to near its usable periphery and is rotated about the target center to produce uniform sputtering of the target and hence sputter deposition on the wafer. The large size of the magnetron can produce fairly uniform target erosion and uniform thickness of the sputtered layer deposited on the wafer.
More recently, however, magnetron sputtering has been extended to deposit thin, nearly conformal layers into high aspect-ratio holes formed in dielectric layers, such as vias for vertical interconnects or trenches for capacitive memories. Examples of such sputtered layers include a barrier layer of, for example, tantalum and tantalum nitride, to prevent migration of metal into the underlying dielectric or a copper seed layer to act as plating electrode and nucleation layer for copper later filled into the via hole by electrochemical plating (ECP). Sputtering into such deep and narrow holes relies in part on a large fraction of sputtered atoms being ionized in a high-density plasma adjacent the target, which can be achieved by a small magnetron which concentrates the target power to a small area of the target, thus producing a high power density and corresponding adjacent high-density plasma region. It has been found that small magnetrons scanned near the periphery of the target effectively can nonetheless produce a nearly uniform sputter deposition over the entire wafer because the sputtered ions diffuse toward the center of the wafer as they travel from the target to the wafer.
However, it is sometimes desired to sputter a wider band on the target with a smaller magnetron. Miller et al. describe a planetary magnetron (PMR) in U.S. Pat. No. 6,852,202, incorporated herein by reference. In the PMR sputter chamber, an inner arm is rotated about the target center and an outer arm spins about an pivot axis at an end of the inner arm and has a magnetron mounted on its end offset from the pivot axis. The described PMR mechanism includes a planetary gear mechanism with a sun gear fixed at the target center and coupled to a gear rotating on the pivot axis and supporting the second arm. The planetary gear mechanism produces a multi-lobed scan pattern in which the radial extent of the scan pattern and the number of lobes is established by the lengths of the two arms and the gear ratio of the gear mechanism. Although this scan pattern has been quite effective in advanced sputtering applications, the lobed scan pattern may not be the optimal one and it is desired to change the scan pattern without changing physical parts of the scan mechanism.